Coupled loop collector circuit



July 14, 1942. w. A. SCHAPER COUPLEl LOOP COLLECTOR CIRCUIT Filed Aug. 7, 1940 (mew/7 .5 /.'s' fa'red famed and r'esamznf near Me /0W fre uencg end .of I/ic band (l/Cuff 1,-5 f/xed ill/26d and resend/1f near The /ow frezueflcy 19/70 Off/1e band ATTORNEY Patented July 14, 1942 COUPLED LooP COLLECTOR CIRCUIT William A. Schaper, Cicero, Ill., assignor to Johnson Laboratories, Inc., Chicago, 111., a corporation of Illinois Application August 7, 1940, SerialNo. 351,755

3 Claims.

This invention relates to high-frequency circuits, such as those employed in radio receiving systems. More particularly, the invention relates to the portion of such systems which constitutes means for collecting the high-frequency signals radiated from relatively distant transmitting stations. This invention incorporates an improved signal-collecting means.

My invention takes advantage of the unique properties of systems which are tuned over a range of frequencies by inductance variation, and makes it possible to employ such systems in a new and highly advantageous manner. One such system is disclosed by Polydorofi in United States Reissue Patent No. 21,282, in which a resonant circuit having an inductance coil and capacitor is adjusted over a range of frequencies by movement of a compressed comminuted core relative- 1y to the inductance coil. This method of tuning is called permeability tuning. An improved form of such a system is disclosed in my United States Patent No. 2,051,012. Both Polydoroffs original system and my improved system readily cover an adequate range of frequencies and may easily be ganged to provide multiple unit systems.

In radio receivers, and more particularly those intended for the reception of broadcasting, it is important that the signal-to-noise ratio at the input to the first vacuum tube be as high as possible. This ratio may be increased either by increasing the amount of signal pick-up or by decreasing the noise pick-up, or by both simultaneously. The signal pick-up. may be improved by increasing the effective dimensions of the signalcollecting means or by tuning the circuit including the signal-collecting means to resonance with the desired signal or both.

One form of collector circuit which includes the signal-collecting means and which may be conveniently tuned to resonance with the incoming signal is that employing an exposed inductive element, commonly called a loop. In such arrangements, both terminals of the exposed inductive element are connected to the remaining elements to form a resonant circuit which is closed through the conductor of the exposed inductive element.

It has been common to tune such closed collector circuits by capacitance variation. In the present application, and in my copending applications, Serial Nos. 319,671, 319,672, 319,673, 319,674 and 319,675, all filed on February 19, 1940, however, I describe closed collector circuits which are tuned by inductance variation, and

preferably by employing an additional unexposed inductor made variable by means of a ferromagnetic element movable relatively to the winding thereof. In my copending applications just referred to, it is explained that this departure from conventional capacitance tuning produces marked advantages because of the control of circuit performance over the tunable frequency range which ferromagnetic inductance variation provides, in spite of the necessarily reduced inductance values which must be employed in the exposed inductive elements. These advantages, as well as additional advantages later to be described, are secured in arrangements according to the present application.

In my copending applications above referred to, I describe a series-tuned collector circuit including a single exposed inductive element which may be a loop, an unexposed variable inductive element and a capacitance. The arrangement according to the present invention, however, employs plural exposed inductive elements forming one fixed-tuned circuit and one variablytuned circuit including a variable unexposed inductive element and a capacitance, and I thus secure plural characteristics which, when properly employed in accordance with the instructions herein given, provide a highly eflicient and compact collector system having excellent uniformity over the frequency range as to both resonant gain and selectivity.

An object of my invention is to provide an improved signal-collecting means for radio receivers.

Another object of my invention is to provide a good signal-to-noise ratio with a relatively small and compact collector.

An additional object is to provide a signal collecting means which may be successfully employed in the most compact forms of radio receivers, and which may be placed in close proximity tothe receiver chassis without serious detriment.

Still another object of the invention is to provide a signal-collecting circuit whose performance characteristics may be readily controlled with respect to variation with frequency.

It is also an object of the invention to provide a signal-collecting circuit which may be tuned by inductance variation, for example by means of a movable ferromagnetic core, and in which the advantages of this method of tuning may be realized.

These and other objects are realized in accordance with my invention in a manner which will be more readily understood by reference to the accompanying drawing, in which:

Fig. 1 is a schematic diagram of the arrangement according to the invention;

Fig. 2 is an equivalent circuit diagram corresponding to the arrangement of Fig. 1;

Fig. 3 is a modification of the arrangement of Fig. l; and

Fig. 4 is a diagrammatic showing of a form of compound loop suitable for use in certain embodiments of the invention.

The arrangement according to Fig. 1 comprises a first exposed inductive element I connected in series with a capacitance 5 to form a fixed-tuned primary closed series resonant circuit grounded at one junction. This primary circuit is loosely coupled inductively to a variably tunable secondary circuit comprising exposed inductive element 2, an unexposed variable inductive element comprising inductive winding 3 and relatively movable ferromagnetic core 3a, and capacitance 8, connected in series in the order named to form a series resonant circuit grounded at its ends. Across the capacitance 8 may be connected the first vacuum tube 9 of a radio receiver, in the conventional manner.

Exposed inductive elements I and 2 preferably take the form of loops, and may be of any physical size consistent with the size of the receiver or other apparatus with which they are to be used. Because of the very high resonant gain secured in accordance with the invention, however, loops I and 2 may be made quite small without detriment, and I shall later describe a construction suitable in the case in which the loops are to be incorporated in a compact receiver.

Exposed inductive elements I and 2 have voltages generated in them by passing signals. The voltage generated in inductive element I is induced in the secondary circuit through the inductive coupling between inductive elements I and 2, and regardless of how inductive elements I and 2 are positioned, the two generated voltages are in additive relation in the secondary circuit. The inductance of unexposed inductor 3 is varied by movement of ferromagnetic core 3a to tune the system for maximum response to any desired signal.

It will be apparent that since the variablytuned circuit includes inductances 2 and 3, the variable inductor 33a must provide greater inductance variation than would be required if the inductance 2 were absent, in order to tune the system over the required frequency range. It may be shown that, if L is the effective external series inductance, p is the ratio of the highest to the lowest frequency in the required tuning range, and La is the minimum inductance of the variable inductor 3-3a, then the required effective permeability of ferromagnetic element 3a is Referring now to Fig. 2, R6 and X1 represent the resistance and inductive reactance respectively of inductor I of Fig. 1, X2 represents the inductive reactance of inductor 2 of Fig. l and X3 represents the inductive reactance of variable inductor 3 of Fig. 1, and R1 represents the resistance of inductors 2 and 3, it being remembered that X3 is variable to tune the system over a range of frequencies, that R7 is also controllably variable by suitable design of ferromagnetic element 3a to control the total effective resistance of the system at all frequencies within the range, and that Re also varies with frequency, but with suitable construction of inductor I may be relatively small and have relatively minor variation. The mutual inductance between inductors I and 2 of Fig. 1, corresponding to reactances X1 and X2, in Fig. 2, is indicated in Fig. 2 by the symbol X4. Capacitive reactance X in Fig. 2 corresponds to capacitor 5 in Fig. 1. Capacitive reactance X8 in Fig. 2

corresponds to capacitor 8 in Fig. 1. The voltages generated in inductors I and 2 of Fig. l by any signal are indicated in Fig. 2 by the symbols E1 and E2 respectively.

The highly advantageous performance of the system depends upon the fact that it possesses two principal characteristics, one due to reactances X2, X3 and X8, and the other due to reactances X1 and X5. By appropriate choice of constants, including the inductance-to-re sistance ratio of variable inductor 3 as it is adjusted to produce resonance over the frequency range, these two characteristics may be caused to produce a desired over-all characteristic for the complete circuit, with high gain throughout the frequency range, as will now be explained.

As is well known, the voltage generated in an exposed conductor by any radio signal is directly proportional to the frequency of the signal, whether the exposed conductor be of the openended antenna type or the closed loop type. Thus if the voltage delivered at the grid of the first vacuum tube of the receiver is to be directly proportional to the strength of the signal, as it preferably should be, the resonant gain of the collector circuit should be inversely proportional to the frequency of the signal. Such a characteristic cannot be secured in a circuit tuned by capacitance variation, because in such systems there is no control of the high-frequency resistance of the circuit and the variation in the resistance of the inductive elements themselves is entirely inadequate to produce the desired result, as experience has shown. Nor is it possible to produce adequate control of the circuit resistance entirely by the action of the ferromagnetic elements in a circuit tuned solely by inductance variation, since these elements inevitably increase the high-frequency resistance as they are inserted into an inductive winding to tune the circuit to the lower frequencies. It is therefore essential that additional means for suitably varying the gain characteristics of the circuit be provided, as contemplated in the present invention.

Again referring to Figs. 1 and 2, the exposed inductive elements I and 2 are fixedly positioned so as to produce a small inductive coupling between them. The resulting coupling reactance, indicated by X4 in the equivalent circuit diagram of Fig. 2, varies directly with frequency. This variation, properly correlated with the control of circuit resistance provided by variable inductor 3, produces the desired relation of voltage at the grid of the tube 9 proportional to the strength but independent of the frequency of the selected signal. It will be apparent that regardless of how inductors I and 2 are connected, the induced voltages due to the coupling reactance X4 and the directly induced loop voltage will be vectorially additive.

The resonant frequency of the fixed-tuned circuit comprising exposed inductor I, and capacitor 5 will be that at which the inductive reactance, X1 minus mutual reactance X4, is equal to the capacitive reactance X5. The voltages E1 and E2 will be proportional to the number of turns in inductors I and 2 respectively. In preferred embodiments, exposed inductor I is arranged to provide increased signal voltage at the lower end of the frequency range, and to this end the fixed-tuned circuit just described is made resonant at a relatively low frequency. Since the permissible inductance of inductor 2 is limited by the available inductance variation of variable inductor 3, and since it is desirable to generate a relatively large signal voltage in inductor I and to employ relative small inductive coupling between inductors I and 2, inductor I preferably has a relatively high inductance value and capacitor is made relatively small. For particular purposes, however, the fixedtuned circuit may be made resonant at any desired frequency by appropriate choice of inductor I and capacitor 5.

Assuming that inductor I and capacitor 5 are such that the fixed-tuned circuit is resonant at or near the lower frequency end of the range, they will have a relatively very small effect upon the resonant frequency of the variably-tuned resonant circuit comprising inductors 2 and 3 and capacitor 8. Although the complete characteristic of the system is complex, as will be indicated by the formulae soon to be given, it may be stated broadly, therefore, that the variably-tuned circuit may be given a desirable performance characteristic by appropriate choice of ferromagnetic element 311, for example, a frequency-voltage characteristic dropping off towards the low frequency end of the band and compensated for by the frequency-voltage characteristic of the fixed-tuned circuit which tends to rise towards the low frequency end of the band.

From the equation between the sum of the voltages E1 and E2 generated by any signal in exposed inductors I and 2 respectively, and the sum of the voltage drops in the circuit, it may be shown that current in the secondary circuit is where N1 and N2 are the turns in inductors I and 2 respectively.

The condition for optimum secondary current is in which Xb= (X2+Xs-Xs) The secondary current for minimum circuit reactance and optimum mutual reactance is The quality factor of the circuit is The resonant frequency, in radians per second is selectivity. If this is done, however, the resonant gain of circuit X2 X3 X8 will not be constant, but will be greater at the higher frequencies. The gain characteristic of fixed-tuned resonant circuit X1 X5 may be such as to compensate for this additional cause of variation, preferably in such a way as to provide a voltage at the grid of tube 9 directly proportional to the strength but independent of the frequency of the selected signal.

Referring now to Fig. 3, the modification here shown differs from the arrangement of Fig. 1 only in that the exposed inductor l is grounded at its midpoint rather than at one of its terminals. This largely eliminates the effect of the capacitive pickup, Which in some cases might be troublesome.

By way of illustrative example of a practical embodiment of the invention having the preferred characteristics described above, the following circuit constants and construction data are given, it being understood that they are not to be taken as in any way limiting the scope of the invention, which resides in the novel circuit arrangement as described in the appended claims, rather than in the particular constants or construction employed.

Exposed inductive elements I and 2 may be single-layer square-section loops having spaced turns and wound on frames 6%" by 6%" by 4%" axial length with No. 28 plain enamelled wire as shown in the following tabulation:

Inductor I:

Turns 16 Axial length of winding; 3.75 Inductance 59.9 ,ch. Q at 600 kilocycles 62 Q at 1560 kilocycles 104 Inductor 2:

Turns 11 Axial length of winding 3.5" Inductance -1; 31.2 h. Q at 600 kilocycles 108 Q at 1560 kilocycles 82 Mutual inductance between inductors I and 2 2.92 n.

Inductor 3 may comprise a progressive universal winding of 15/44 single silk enamelled litz wire 11%" long on an insulating tube of 0.205" inside diameter and 0.221" outside diameter, and having an inductance of 47.2 [.Lh., a Q of 111 at 600 kc., and a Q of 83 at 1560 kc.

Ferromagnetic element 311 may comprise a compressed comminuted core of hydrogen-reduced powdered iron that has been sifted through a screen having 400 meshes to the inch. For use with the above described inductor, it may be 0.200" in diameter and 1%" long, being preferably hot-molded at F. with 0.5% particle insulation and 3% of powdered Bakelite binder, and cured at 290 F. for 3 hours. Such a core will have an effective permeability of about 11.5, so that the maximum inductance of variable inductor 33a will be of the order of 550 microhenrys. For use with the above described inductors, capacitors '5 and 8 may be of 1477 d. and 134 LL/.Lf. capacitance respectively. With these constants the fixed-tuned circuit will be resonant at approximately 600 kc. and the variably-tunable circuit will cover the range from 540 kc. to 1560 kc. The gain ratio of the system will be of the order of 0.56 at the lowfrequency end of the range and of the order of 0.702 at the high-frequency end of the range.

Fig. 4 shows a desirable construction for inductors l and 2 when they take the form of small loops such as those just described. The forms I0, It may be identical and may suitably be constructed of hard wood, or, if desired, a molded form of insulating material may be used. The required small degree of coupling is secured by placing the loops substantially in the same plane, as shown, with their adjacent edges approximately 1" apart.

When used in connection with a radio receiver of the superheterodyne type having an intermediate frequency of the order of 460 kc., my improved collect-or circuit provides exceptionally high discrimination against signals of the socalled image frequency. In the illustrative embodiment above described, for example, the image ratio will be of the order of 89 at the highfrequency end of the range and of the order of 890 at the low-frequency end of the range. This is due to the fact that while inductor l provides a very considerable portion of the total pickup at the lower frequencies where the image response is most troublesome, the fixed-tuned circuit including inductor l is very far from resonance with the image signals and thus very efiectively excludes them. This high image ratio, therefore, is an inherent and highly advantageous attribute of my invention.

Having thus described my invention, what I claim is:

1. A signal-collecting system for use in radio receivers having a first vacuum tube with an input electrode and tunable over a range of frequencies, including a closed fixed-tuned resonant circuit comprising a first exposed inductive element and a capacitor in series and resonant near the low-frequency end of said range, a variablytuned resonant circuit connected to said input electrode and comprising a second exposed inductive element, a second capacitor and an unexposed inductive winding in series, said exposed inductive elements being positioned to produce a small degree of inductive coupling between said circuits, said fixed tuned resonant circuit being coupled to said vacuum tube solely by said inductive coupling, and means for tuning said tunable circuit to resonance with any desired signal within said range solely by variation of the inductance of said unexposed inductive winding.

2. A signal-collecting system for use in radio receivers having a first vacuum tube with an input electrode and tunable over a range of frequencies, including a closed fixed-tuned resonant circuit comprising a first exposed inductive element and a capacitor in series and resonant near the low-frequency end of said range, a variablytuned resonant circuit connected to said input electrode and comprising a second exposed inductive element, a second capacitor and an unexposed inductive winding in series, said exposed inductive elements being positioned to produce a small degree of inductive coupling between said circuits, said fixed tuned resonant circuit being coupled to said vacuum tube solely by said inductive coupling, and means including a ferromagnetic element movable relatively to said unexposed inductive winding for tuning said tunable circuit to resonance with any desired signal within said range solely by variation of the inductance of said unexposed inductive winding.

3. A signal collecting system for use in radio receivers having a first vacuum tube with an input electrode and tunable over a range of frequencies, including a closed fixed tuned resonant circuit comprising a first exposed inductive element and a capacitor in series, a variably tu ned resonant circuit connected to said input electrode and comprising a second exposed inductive element, a second capacitor, an unexposed inductive winding in series and a ferromagnetic element movable relative to said winding for tuning said second circuit to resonance with any desired signal within said range of frequencies, said exposed inductive elements being positioned to produce a small degree of inductive coupling between said circuits, said fixed tuned resonant circuit being coupled to said vacuum tube solely by said inductive coupling and being resonant near the low frequency end of said frequency range to provide increased signal voltage at the lower end of said range, and said ferromagnetic element being of a construction to impart to said variably tuned circuit a dropping voltage characteristic with decreasing frequency, so that the overall voltage supplied to the input electrode of said tube is directly proportional to the strength but independent of the frequency of the selected signal.

WILLIAM A. SCHAPER. 

